Vibrational analysis of amino acids and short peptides in hydrated media. II. Role of KLLL repeats to induce helical conformations in minimalist LK-peptides

Aqueous solution secondary structures of minimalist LK-peptides, with the generic sequence defined as KLL(KLLL)nKLLK, have been analyzed by means of circular dichroism (CD) and Raman scattering techniques. Our discussion in the present paper is mainly focused on four synthetic peptides (from 5 to 19 amino acids), KLLLK, KLLKLLLKLLK, KLLKLLLKLLLKLLK, and KLLKLLLKLLLKLLLKLLK, corresponding to the repeat unit, and to the peptide chains with the values of n = 1-3, respectively. CD and Raman spectra were analyzed in order to study both structural features of the peptide chains and their capability to form aggregates. On the basis of the obtained results it was concluded that the conformational flexibility of the shortest peptides (5-mer and 11-mer) is high enough to adopt random, beta-type, and helical chains in aqueous solution. However, the 11-mer shows a clear tendency to form beta-strands in phosphate buffer. The conformational equilibrium can be completely shifted to beta-type structures upon increasing ionic strength, i.e., in PBS and tris buffers. This equilibrium can also be shifted toward helical chains in the presence of methanol. Finally, the longest peptides (15-mer and 19-mer) are shown to form alpha-helical chains with an amphipathic character in aqueous solution. The possibility of bundle formation between helical chains is discussed over the temperature-dependent H-D exchange on labile hydrogens and particularly by considering the particular behavior of an intense Raman mode at 1127 cm-1 originating from the leucine residue side chain. The conformational dependence of this mode observed upon selective deuteration has never been documented up to now.     

Kinetic and thermodynamic control in the cyclization via thiiranium ions. Stereoselective synthesis of a 2,3,5‐trisubstituted tetrahydropyran ring

The stereoselective rearrangement of tetrahydrofuran or tetrahydropyran rings having a phenylsulfanyl group in an exo position, via the intermediate thiiranium ions, is reported. The 5‐ or 6‐exo‐tet cyclization of hydroxy sulfides gave the kinetic products while the 6‐endo‐tet or 5‐endo‐tet gave the thermodynamic products. The rearrangement of the 5‐exo product to the 6‐endo‐ one is an interesting way for the stereoselective synthesis of substituted tetrahydropyrans.     

Determination of zinc in pharmaceutical products using an electrodialyser incorporated into a flow system

An electrodialyser unit has been designed, incorporated into the conduits of a flow injection (FI) system, and employed for the removal of interferences in the determination of zinc(II) in multivitamin tablets. The proposed system is fully computerised and is able to determine zinc(II) at a frequency of 18 samples/h with a sample interaction of 0.0015%. The calibration graph is linear between 1 and 20 mg/L. The detection limit is 0.998 mg/L. The results obtained for zinc(II) in multivitamin tablets with the proposed system compared favourably with those obtained with the standard manual AAS method.     

Determination of zinc in pharmaceutical products using an electrodialyser incorporated into a flow system

An electrodialyser unit has been designed, incorporated into the conduits of a flow injection (FI) system, and employed for the removal of interferences in the determination of zinc(II) in multivitamin tablets. The proposed system is fully computerised and is able to determine zinc(II) at a frequency of 18 samples/h with a sample interaction of 0.0015%. The calibration graph is linear between 1 and 20 mg/L. The detection limit is 0. 998 mg/L. The results obtained for zinc(II) in multivitamin tablets with the proposed system compared favourably with those obtained with the standard manual AAS method.     

Synthetic studies on statins. Part 1: a short and cyanide-free synthesis of atorvastatin calcium via an enantioselective aldol strategy

A short and cyanide-free enantioselective synthesis of atorvastatin calcium has been achieved starting from a commercially available highly substituted 1,4-diketone in an overall yield of 40%. The key step in this approach is the asymmetric aldol reaction of an aldehyde with diketene in the presence of Ti(O-i-Pr)₄–Schiff base complex to create the (5R)-stereochemistry of atorvastatin calcium.     

A rare occurrence of a β‐turn in an amyloid βA4 peptide

The fragment β(25–35) of the amyloid β‐peptide, like its parent βA4, has shown neurotrophic and late neurotoxic activities in cultured cells. The 3D structure of this important peptide was examined by ¹H and ¹³C 2D‐NMR and MD simulations in DMSO‐d₆ and water. The NMR parameters of chemical shift, ³J(N,Hα) coupling constants, temperature coefficients of NH chemical shifts and the pattern of intra and inter‐residue NOEs were used to deduce the structures. In DMSO‐d₆, the peptide was found to take up a type I β‐turn around the C‐terminal residues Ile⁸–Gly⁹–Leu¹⁰–Met¹¹, whereas in water at pH 5.5, it adopts a random coil conformation. This is only the second report of a β‐turn in the β‐amyloid class of peptides. The solution structures generated using restrained molecular dynamics were refined by MARDIGRAS to an R factor of 0.33 in the case of DMSO‐d₆ and to 0.56 for water. Copyright © 2002 John Wiley & Sons, Ltd.     

Kinetic analysis of distribution of weak acid in an oil-gelatin microcapsule/water system studied by microcapillary injection and microabsorption methods.

The distribution of anthracene-9-carboxylic acid across dibutyl phthalate/gelatin-membrane/water interface of a single microcapsule was analyzed using microcapillary manipulation and microabsorption methods. The partitioning ratio and the distribution rate in the microcapsule/water system were measured for various pH values in the water phase. Results were compared with those in the dibutyl phthalate/water system in the absence of the gelatin membrane. The distribution rate could be analyzed on the basis of a first-order type reaction. The observed rate constant was linearly proportional to the inverse of the microcapsule radius, indicating that the distribution rate is limited by interfacial mass transfer. Analysis of the pH dependence of the interfacial mass transfer rate suggests that the mass transfer of the neutral species of anthracene-9-carboxylic acid (AnH) competes with the ion transfer of the dissociated species (An-) at the liquid/liquid interface in the gelatin membrane of the microcapsule.     

Metallaborane reactivity. A stoichiometric mechanism for the insertion of two alkynes into an iridaborane framework via a disposable molybdenum chaperone

Building on earlier work that showed the formation of [1-Cp*-2,2,2-(CO)3-2-THF-nido-1,2-IrMoB(4)H(8)], 2, from the reaction of [1-Cp*-arachno-1-IrB(4)H(10)], 1, with (arene)Mo(CO)3, the stoichiometric mechanism for the generation of [1-Cp*-5,6,7,8-(R)4-nido-1,5,6,7,8-IrC(4)B(3)H(3)], 8, from the reaction of 2 with RCCR, R = Me, Ph, has been identified. For R = Me, the major product in solution is [1-Cp*-5,6,7,8-(CH3)4-closo-1,5,6,7,8-IrC(4)B(3)H(3)Mo(CO)3], 7, which is in equilibrium with 8. The equilibrium 8 + Mo(THF)3(CO)3 <==> 7 + 3THF is characterized by DeltaH = 8 kcal/mol and DeltaS = 34 cal/mol K. Density functional theory calculations carried out on 7 indicate that the Mo(CO)3 moiety is weakly bound to the cluster mainly through Mo-C rather than Mo-B interactions. Under alkyne deficient conditions, the product [1-Cp*-2,2,2-(CO)3(mu-CO)-3,4-(CH3)2-closo-1,2,3,4-IrMoC(2)B(3)H(3)], 6, can be isolated. Solid-state structures of 1 and 2 have been reported previously, and those of 6, 7, and 8, R = Me, Ph, are reported here. The evolution of products with time was monitored by 1H and 11B NMR and showed the formation and decay of two additional species which have been identified as the structural isomers [1-Cp*-7,7,7-(CO)3-7-THF-2,3-(CH3)2-nido-1,7,2,3-IrMoC(2)B(3)H(5)], 4, and [5-Cp*-7,7,7-(CO)3-7-THF-2,3-(CH3)2-nido-5,7,2,3-IrMoC(2)B(3)H(5)], 5, with the metals nonadjacent in 4 and adjacent in 5. Circumstantial evidence suggests that 4 is the precursor to 5 and 5 is the precursor to both 6 and 7. Cluster 2 also is a catalyst or catalyst precursor for the isomerization of olefins, namely, hex-1-ene to cis-hex-2-ene and tetramethyl allene to 2,4-dimethylpenta-1,3-diene. These novel results also establish that [1-Cp*-2,2,2-(CO)3-2-(alkyne)-nido-1,2-IrMoB(4)H(8)], 3, forms from 2 and constitutes a logical precursor to 4. The entire process, 1 + 2alkyne = 8 + BH3 + 2H2, which is promoted by (arene)Mo(CO)3, constitutes an explicit example of a transition-metal-facilitated process analogous to metal-facilitated organic transformations observed in organometallic chemistry.     

Peptide-lipid interactions in Langmuir monolayers at the air/water interface. A novel thermodynamic analysis of a two-component surfactant system

We have recently developed a useful approach to determine the possible desorption of largely insoluble one-component monolayers into the aqueous subphase. In the present paper the underlying theoretical line of reasoning will be extended to the more general case of two-component monolayers. A practical application is demonstrated for the special system of a biomembrane fusion peptide that interacts with a phospholipid monolayer under equilibrium conditions. Through a pertinent evaluation of conventional surface pressure/area data the actual amount of peptide incorporated (relative to the amount of interfacial lipid) and its true molecular area can each be determined individually without additional information. Both quantities are shown to undergo a very pronounced reduction upon raising the lateral pressure from 10 to 25 mNm⁻¹.     

Kinetic and mechanistic studies of the reactivity of iron(IV) TAMLs toward organic sulfides in water: resolving a fast catalysis versus slower single-turnover reactivity dilemma

TAML complex is oxidized by H₂O₂ or ^tBuOOH in water at pH < 10 into the corresponding iron(IV) μ-oxo-bridged dimer 2, which oxidizes readily ring-substituted thioanisoles p-XC₆H₄SMe (X=H, MeO, Me, Cl, CN) into the corresponding sulfoxides with regeneration of 1. The oxidation studied under pseudo-first-order conditions using the stopped-flow technique by monitoring the fading of the 420-nm band of 2 follows hyperbolic kinetics according to the rate law k_obs = ab[p-XC₆H₄SMe]/(1 + b[p-XC₆H₄SMe]) at pH 8 and 25 °C. Parameters a, b, and ab all decrease for electron-poorer thioanisoles and the Hammett value ρ?~?1 has been found for ab, which can be associated with the second-order rate constants for oxidation of thioanisoles by 2. The kinetics of oxidation of p-NO₂C₆H₄SMe by H₂O₂ catalyzed by 1 has been studied under steady-state conditions. Covering the concentration of 1 in a 100-fold range has revealed that though first-order kinetics in 1 is observed at low catalyst concentrations (below 10^?6 M), there is a significant negative deviation from linearity at [1]?>?10^?6 M. The latter was rationalized by the equilibrium between the monomeric and dimeric Fe^IV species 2 M???M–M (K_d), both being able to oxidize p-NO₂C₆H₄SMe with rate constants k_m and k_d which were found to be (13?±?1)?×?10⁴ and (0.32 ± 0.01)?×?10⁴ M^?1 s^?1, respectively. The difference in the rate constants is the key for resolving the dilemma of faster catalysis versus slower single-turnover reactivity of TAML activators in water.     

Kinetic parameters relevant to repetitive determinations via transient signals in flow-through systems : Part I. Direct Sample Injection into the Detection Chamber with application to the determination of some phenothiazines by a transient redox effect

The effect of rate proportionality constants on three fundamental parameters, which limit sensitivity as well as determination rate in sample injection (for repetitive determinations) into unsegmented (single-phase) flow-through systems, is discussed. Under first order or pseudo-first order conditions, the individual values and ratios of the rate constants for the two consecutive processes leading to the transient signal control the three parameters; namely : (1) the ratio of peak height to theoretical steady-state signal, (2) the time to reach the maximum (or minimum) signal value, and (3) the time to return to baseline. A simple way of extracting the values of the rate constants from the signal profiles when the two processes cannot be isolated is presented. Application to the determination of some phenothiazines based on the transient redox effect observed in their oxidation with cerium(IV) is also illustrated. The discussion and application are limited to the case of direct sample injection into the detection chamber.     

Influence of water on protein structure. An analysis of the preferences of amino acid residues for the inside or outside and for specific conformations in a protein molecule

The x-ray structures of 20 proteins have been examined and each of the residues in these proteins was assigned to the inside or outside of the molecules and to a conformational state. The data obtained confirm that polar groups are generally found on the outside of proteins and nonpolar residues are generally found on the inside. Seven of the amino acids (Ala, Arg, Cys, His, Pro, Ser, Tyr) have inside/outside preferences which are not consistent with their usual assignment as either polar or nonpolar residues; explanations are given for these apparent inconsistencies. Of the three types of backbone structure considered here (extended, alpha helix, and nonregular), extended structures have the greatest preference for the inside of proteins, and nonregular structures have the greatest preference for the outside. It is suggested that differences in entropy play an important part in the inside/outside preferences of backbone structures. There are generally significant changes in the conformational preferences of the residues in going from the inside to the outside of proteins; environmental (rather than local) solute-solvent interactions seem to be the predominant cause of these changes in conformational preferences.     

Effect of small molecule surfactants on molecular parameters and thermodynamic properties of legumin in a bulk and at the air–water interface depending on a protein structure in an aqueous medium

This paper presents the effect of fatty acid salts, namely, Na-caprate and Na-palmitate on the legumin (11S globulin of Vicia Faba broad beans) molecular and thermodynamic properties in the bulk aqueous medium and at the air–water interface under different molecular states of the protein. That are the native state of the protein globule (pH 7.2, ionic strength of 0.05 mol dm⁻³), as well as the acidic denatured (pH 3.0, ionic strength of 0.01 mol dm⁻³) and the heat denatured ones (after heating at 90°C for 30 min, pH 7.2, ionic strength of 0.05 mol dm⁻³). In turn, an importance of the state of the small molecule surfactants in a solution in reference to the critical concentrations of micelle formation (CMC), for their effect on the protein properties, was also under our studying. The peculiarities of the legumin structure in the aqueous medium appeared in the different nature of the interactions between the protein and the fatty acid salts, as was indicated by the mixing calorimetry data. So, the hydrophobic contacts provided a basis for interactions between both the native and heat denatured legumin with the fatty acid salts. At the same time, the electrostatic interactions between the oppositely charged functional groups of the fatty acid salts and the acidic denatured protein formed principally a basis of their interactions in an aqueous medium. In response to interactions of the fatty acid salts with legumin the essential changes in the protein conformational stability, depending on both the protein molecular state and concentration of the fatty acid salts, were found using differential scanning calorimetry (DSC). The rather high level of the protein association was observed by light scattering in the bulk aqueous medium in the presence of the fatty acid salts. As this takes place, the surface hydrophilicity of the protein increased under the formation of the associates. The combined data of mixing calorimetry, differential scanning calorimetry and light scattering suggested the complex formation between legumin and the fatty acid salts. The interactions of the fatty acid salts with the protein produced a change in the surface activity for the mixture of the protein with the fatty acid salts. That is a decrease in the protein surface tension at the air–water interface for the mixed solutions in comparison with ones for both the protein and small molecule surfactant alone in the case of Na-caprate, and those are the intermediate values of the surface tension in the case of Na-palmitate. These results were observed independently of the protein state (native or acidic/heat denatured) in an aqueous medium. As this took place, the most dramatic increase in the surface activity was found for the mixtures of the acidic denatured protein with Na-caprate as if the most hydrophobic species were formed in this case. The combined data of mixing calorimetry, DSC, light scattering and tensiometry showed that the effect of the fatty acid salts on the legumin thermodynamic properties in a bulk and at interfaces is governed by a number of the key factors such as: a structure of both the protein and fatty acid salt (a length of the hydrocarbon chain); a degree of the protein association in the bulk aqueous phase (as a result of the interactions with the small molecule surfactants); a change in the protein conformational stability (flexibility) under the influence of the small molecule surfactants; as well as by the nature (hydrophobic, electrostatic) of the protein–small molecule surfactant interactions, determining ultimately the hydrophilic–lipophilic balance of the protein surface.     

Structure-selective dye uptake into an aggregate of a copolymer with linear polyelectrolyte block and hydrophobic block carrying pendant dendritic moiety in water

Selective uptake of various dyes into an aggregate of amphiphilic copolymer consisting of a hydrophilic linear polyelectrolyte block and hydrophobic block carrying pendant dendritic moiety has been investigated in water. The copolymer associated into an aggregate with a hydrophobic interior at concentrations above 0.2 mg cm(-3). The uptake (23 and 36 molecules per aggregate, respectively) of pyrene and Oil Yellow in an aggregate was one order higher than that of benzo[a]pyrene and SudanIII. The hydrophobic dyes are always doped in the interior of the aggregate, but the difference in uptake among dyes may depend on their structure. Even if a large number of guest molecules was doped into the interior of an aggregate, the size of the sphere-like aggregate was conserved. It is suggested that guest molecules are encapsulated into the persisting cavity within and between hydrophobic dendron moieties in an aggregate. Structure-selective uptake reported in this investigation is a unique character of an aggregate of copolymer with dendron moiety because the dendron moiety offers a large void for doping.     

Quantum chemical studies of a model for peptide bond formation. 3. Role of magnesium cation in formation of amide and water from ammonia and glycine

The SN2 reaction between glycine and ammonia molecules with magnesium cation Mg2+ as a catalyst has been studied as a model reaction for Mg(2+)-catalyzed peptide bond formation using the ab initio Hartree-Fock molecular orbital method. As in previous studies of the uncatalyzed and amine-catalyzed reactions between glycine and ammonia, two reaction mechanisms have been examined, i.e., a two-step and a concerted reaction. The stationary points of each reaction including intermediate and transition states have been identified and free energies calculated for all geometry-optimized reaction species to determine the thermodynamics and kinetics of each reaction. Substantial decreases in free energies of activation were found for both reaction mechanisms in the Mg(2+)-catalyzed amide bond formation compared with those in the uncatalyzed and amine-catalyzed amide bond formation. The catalytic effect of the Mg2+ cation is to stabilize both the transition states and intermediate, and it is attributed to the neutralization of the developing negative charge on the electrophile and formation of a conformationally flexible nonplanar five-membered chelate ring structure.     

Encapsulation of an Ionic Metalloporphyrin into a Zeolite Imidazolate Framework in situ for CO2 Chemical Transformation via Host–Guest Synergistic Catalysis

It is a great challenge to rationally integrate multiple reactive sites into a composite material with confined nanospace, which can be applied as a nanoreactor to facilitate targeting catalytic reaction. In this work, an ionic metalloporphyrin has been encapsulated in situ into ZIF‐8 for a solvent‐free synthesis of cyclic carbonates from CO₂ and epoxides without any co‐catalyst under 1 atm CO₂.     

Indium(III) iodide-mediated Strecker reaction in water: an efficient and environmentally friendly approach for the synthesis of α-aminonitrile via a three-component condensation

A mild, efficient and environmentally friendly method has been developed for the synthesis of α-aminonitriles via a three-component condensation of aldehyde, amine and TMSCN in the presence of a catalytic amount of indium(III) iodide in water. The reactions proceeded smoothly at room temperature in water to generate the corresponding products in moderate to excellent yields.